Ferrite is a generic term for any compound including an oxide of a divalent cationic metal and trivalent iron, and ferrite magnets have found a wide variety of applications in numerous types of rotating machines, loudspeakers, and so on. Typical materials for a ferrite magnet include Sr ferrites (SrFe12O19) and Ba ferrites (BaFe12O19) having a hexagonal magnetoplumbite structure. Each of these ferrites is made of iron oxide and a carbonate of strontium (Sr), barium (Ba) or any other suitable element, and can be produced at a relatively low cost by a powder metallurgical process.
A basic composition of an (M-type) ferrite having the magnetoplumbite structure is normally represented by the chemical formula AO.6Fe2O3, where A is a metal element to be divalent cationic and is selected from the group consisting of Sr, Ba, and other suitable elements.
It is known that the remanence Jr of a Ba ferrite or an Sr ferrite can be increased by substituting a rare earth element such as La for a portion of Ba or a portion of Sr and by substituting Zn for a portion of Fe (see Journal of Magnetism and Magnetic Materials, Vols. 31-34 (1983), 793-794, Japanese Patent Application No. 8-145006 and Japanese Laid-Open Publication No. 9-115715).
It is also known that the coercivity HcJ and Jr of a Ba ferrite or an Sr ferrite can be increased by substituting a rare earth element such as La for a portion of Ba or a portion of Sr and by substituting Co for a portion of Fe (see Bull. Acad. Sci. USSR (Tranl.) phys. Sec. vol. 25 (1961) 1405-1408, Japanese Patent Application No. 8-306072 and Japanese Laid-Open Publication No. 10-149910).
As for an Sr ferrite, it was also reported that HcJ and Jr thereof can be increased by substituting La for a portion of Sr and by substituting Co and Zn for a portion of Fe (see PCT International Application No. PCT/JP98/00764 (corresponding to PCT International Publication No. WO 98/38654).
Meanwhile, it was also proposed that La and Co, which are rather expensive materials, be added in small amounts to an Sr ferrite. For example, Japanese Laid-Open Publication No. 11-307331 reports that when 0.05 mol or less of La, 0.05 mol or less of Co and Mn were added to an Sr ferrite, the loop squareness of the B—H curve thereof improved.
There was a report that the magnetic properties improved when 1.5 wt % to 4 wt % of La2O3, for example, was added (see Japanese Laid-Open Publication No. 1-283802).
It was further reported that when 0.05 wt % to 5 wt % of CaO, SiO2, CoO, Cr2O3, Al2O3, SrO or BaO was added to a Ba ferrite or an Sr ferrite, the magnetic properties thereof improved (see Japanese Laid-Open Publication No. 5-42128 and Japanese Patent No. 2908631).
However, none of these ferrite magnets cannot improve the magnetic properties sufficiently and reduce the manufacturing cost significantly at the same time. Specifically, it was reported that if La was substituted for a portion of Ba or a portion of Sr and if Zn was substituted for a portion of Fe in a ferrite, the ferrite exhibited increased Jr. In that case, however, HcJ thereof decreased noticeably.
Also, it was reported that when La was substituted for a portion of Ba or a portion of Sr and when Co was substituted for a portion of Fe in a ferrite, the ferrite exhibited increased HcJ. However, Jr of the ferrite did not increase so much as in the ferrite in which La was substituted for a portion of Ba or a portion of Sr and in which Zn was substituted for a portion of Fe.
It was reported that the Sr ferrite exhibited increased Jr and increased HcJ when La was substituted for a portion of Sr and when Co and Zn were substituted for a portion of Fe. However, such an Sr ferrite easily causes an excessive grain growth during the sintering process thereof and eventually has decreased HcJ often.
Furthermore, if a rare earth element (such as La) and Co are used in large amounts as substituents for a ferrite, then the material cost of such a ferrite increases adversely because the raw materials of these substituents are expensive. In that case, the essential feature of the ferrite magnet, which should be produced at a lower manufacturing cost than a rare earth magnet, for example, might be lost.
There is also a proposal that those expensive La and Co be added in small amounts. Nevertheless, if that La or Co added is as small as 0.05 mol or less, then neither Jr nor HcJ can be increased significantly although Jr and HcJ are two of the most important magnetic properties.
Furthermore, even if only La2O3 is added or if CaO, SiO2, CoO, Cr2O3, Al2O3, SrO and/or BaO are added in combination, the magnetic properties improve just slightly.
To produce a ferrite at a low cost, the compaction process, which is normally a costly manufacturing processing step, needs to be carried out in a short time. A dehydration process accounts for most of the compaction process time. To shorten the time it takes to carry out this dehydration process, a powder needs to be finely pulverized into particles with a relatively large size in a fine pulverization process, which should be performed before the compaction process. However, if the size of those fine particles is increased, then Jr and HcJ decrease.
Furthermore, the degree of alignment and the density of a powder should be raised to increase Jr. In particular, the density may be increased if the powder is sintered at an elevated temperature. However, the high-temperature sintering process may cause an excessive grain growth and decreased HcJ.
In order to overcome the problems described above, a primary object of the present invention is to provide a ferrite magnet that can be produced at a low manufacturing cost and that can still exhibit improved magnetic properties and a method of making such a ferrite magnet. A more specific object is to minimize the decrease in coercivity even when the sintering process is carried out at an elevated temperature.